1. Field of the Invention
This invention generally relates to electroluminescent display technology, and more particularly to an electroluminescent display that can reduce the power consumed while driving the pixel array.
2. Description of the Related Art
Electroluminescent display technology has recently attracted many researches and developments in the field of emissive displays. Compared to other types of emissive displays such as the plasma display, the electroluminescent display promises advantages such as lower power consumption, reduced size, and high image brightness and sharpness. An electroluminescent display system conventionally includes a mesh of scan and data lines that define an array of pixels in each of which is coupled one electroluminescent or light-emitting device. The light-emitting device particularly can be an organic light-emitting device (OLED), and is usually driven by a driving circuit associated to each pixel.
Conventionally, a basic OLED cell is constructed from a stack of layers made of organic material and sandwiched between two electrode layers, i.e. one anode and one cathode. The organic layers are configured to form functional layers usually including a hole transport layer, an emissive layer, and an electron transport layer. When an adequate voltage is applied between the anode and the cathode, the injected positive and negative charges recombine in the emissive layer to produce light.
FIG. 1A is a schematic view of a conventional pixel driving circuit implemented in an organic electroluminescent display known in the art. The pixel driving circuit 110 includes two transistors 112, 114, a storage capacitor 116, and an organic light-emitting diode 118. The transistors 112, 114 can be any types of transistor, such as PMOS thin film transistors or the like. The transistor 112 works as a switch and includes a gate connected to a scan line SCAN, and a source connected to a data line DATA, and a drain connected to the storage capacitor 116. The transistor 114 works as a current driver and includes a source connected to the anode of the organic light-emitting diode 118, while its drain is connected to a positive voltage terminal PV. The storage capacitor 116 is coupled between the gate and the drain of the transistor 114. The cathode of the organic light-emitting diode 118 is connected to a ground potential.,
In this conventional circuit scheme, the voltage bias applied between the terminal PV and the ground potential usually results in a gate voltage of the driving transistor 114 between about +4.5V and +6.5V to have its operating in the saturation range for delivering an electric current to the organic light-emitting diode 118. This constitutes a relatively high power consumption that requires specific manufacture techniques to construct a reliable driving circuitry.
FIG. 1B illustrates another pixel driving circuit known in the art. This pixel driving circuit is disclosed in U.S. Pat. No. 6,509,692 issued to Komiya, the entire disclosure of which is incorporated herein by reference. The pixel driving circuit shown in FIG. 1B is very similar to that of FIG. 1A, except that the power source includes a positive voltage terminal PV and a negative voltage terminal CV between both of which are coupled the driving transistor 114 and the organic light-emitting diode 118.
This configuration of the power source enables to reduce the operating gate voltage of the driving transistor 114 down to a voltage range between about 3V and 0.5V. As a result, the driving circuitry can be constructed with less expensive CMOS techniques and operate with a lower power consumption.
FIG. 1C is a general diagram of a power generator circuit conventionally implemented to provide the power source of FIG. 1B. Conventionally, two power circuits including two DC/DC converters 130 are required to convert an initial voltage V to positive and the negative voltage potentials PV, CV. As a result, the manufacture cost is usually increased for this type of power source configured with both positive and negative voltage potentials. Further, the conversion efficiency of the DC/DC converter 130 usually is about 80%, in other words undesirable energy dissipation occurs in the power source. In addition, the installation of two DC/DC converters 130 increases the ripple factor, which affects the image quality of the display system. The foregoing and other disadvantages call for improvements of the power source in the pixel driving circuit.
Therefore, there is presently a need for an electroluminescent display, and in particular a pixel driving circuit that can overcome the disadvantages related to the power source.